WASHINGTON — The Florida congressman who succeeded Mark Foley after he resigned because of a sex scandal is now embroiled in a sex scandal of his own, and has requested a Congressional ethics investigation to clear his name.The congressman, Tim Mahoney, a Democrat, agreed to a $121,000 settlement with a former mistress who worked on his staff and was threatening to sue him, two Democratic staff members who have been briefed on the settlement said.The revelation, first reported by ABC News, could cost Mr. Mahoney his House seat. His South Florida district is conservative, and he was already in one of the most competitive races involving an incumbent Democrat.Mr. Mahoney was elected two years ago after the resignation of Representative Foley, a Republican, whose lewd Internet messages to Congressional pages created a national outrage.Without denying the accusations or explaining how he might benefit from an ethics investigation, Mr. Mahoney said the truth would vindicate him.“While these allegations are based on hearsay, I believe that my constituents need a full accounting,” Mr. Mahoney said in a written statement. “As such, I have requested the House Ethics Committee to review these allegations.”Speaker Nancy Pelosi also called Monday for a House ethics inquiry.The money, which did not come from campaign funds, was paid as part of a severance package to the aide, Patricia Allen, after she was fired, a spokeswoman for Mr. Mahoney said.Mr. Mahoney’s wife was aware of the affair and the legal agreement involving Ms. Allen’s dismissal, two Democratic staff members said.That agreement included a $61,000 severance payment, a $60,000 payment for legal fees and an agreement by Ms. Allen not to join any other campaign for at least a year, the staff members said. They said the agreement also involved a promise to Ms. Allen of a job at Fletcher Rowley Chao Riddle, a Nashville firm that handled Mr. Mahoney’s advertising.On Monday, the company said that it was severing all ties with Mr. Mahoney’s campaign and that it had no knowledge of the promises to Ms. Allen for employment.The affair began in 2006, when Mr. Mahoney was running for Congress, Democratic staff members said. Mr. Mahoney first met Ms. Allen at a campaign stop and later arranged for her to volunteer for the campaign, the Congressional aides said.Friends of Ms. Allen told ABC that she had sought to break off the affair when she learned that Mr. Mahoney was involved in other extramarital relationships.Mr. Mahoney fired Ms. Allen in January 2008, and she began legal proceedings in February that concluded with a settlement in March, Democratic staff members briefed on the matter said.“You work at my pleasure,” Mr. Mahoney told Ms. Allen in a Jan. 20 telephone call that was recorded and played for Mr. Mahoney’s employees. “If you do the job that I think you should do, you get to keep your job. Whenever I don’t feel like you’re doing your job, then you lose your job. And guess what: The only person that matters is guess who? Me.”In 2007, rumors began spreading on Capitol Hill that Mr. Mahoney was having an affair. Representative Rahm Emanuel of Illinois, one of several senior Democrats who heard the rumors, approached Mr. Mahoney to discuss the matter.“Congressman Emanuel confronted Congressman Mahoney, told him he was in public life and had a responsibility to act accordingly and appropriately, and urged him to do so,” said Sarah Feinberg, a spokeswoman for Mr. Emanuel. “They had no further conversations on this topic.”Mr. Mahoney is facing Tom Rooney, a lawyer, who was trailing Mr. Mahoney by about seven percentage points in a September poll.